(1) Field
The field of the present invention relates to devices and methods for generating light, more particularly to the field of plasma lamps, and still more particularly to plasma lamps driven by a radio-frequency source without the use of internal electrodes or surrounding dielectric bodies that enhance electromagnetic field coupling. Additionally, the field of the present invention relates to devices and methods where the lamp is not incorporated into or a subset of a microwave resonator, a cavity or a waveguide, in particular the lamp and resonator or cavity structure are not geometrically contiguous.
(2) Related Art
Plasma lamps provide extremely bright, broadband light, and are useful in applications such as projection systems, industrial processing, and general illumination. The typical plasma lamp manufactured today contains a gas-fill or mixture of a noble gas (Argon, Xenon, or other) and trace amounts of a light emitter that are excited to form a plasma. Plasma interaction with the light emitter (e.g. Sodium and Mercury) gives rise to light in the UV, visible, and near infrared portions of the electromagnetic spectrum. Gas ionization resulting in plasma formation is accomplished by passing a high-current through closely-spaced electrodes contained within the vessel that houses the plasma. This arrangement, however, suffers from electrode deterioration due to sputtering of the metal electrodes, and therefore exhibits a limited lifetime.
Electrode-less plasma lamps driven by microwave sources overcome the lifetime issue related to electrode deterioration and have been disclosed in the prior art. For example, both U.S. Pat. No. 6,617,806B2 (Kirkpatrick et. al.) and US Patent Application Number US2001/0035720A1 (Guthrie et. al.) disclose similar basic configurations of a gas-fill encased either in a bulb or a sealed recess within a dielectric body forming a waveguide, with microwave energy being provided by a source such as a magnetron and introduced into the waveguide and heating the plasma resistively. U.S. Pat. No. 6,737,809B2 (Espiau et. al.) discloses a somewhat different arrangement whereby the plasma-enclosing bulb is contiguous with a dielectric resonator and forms part of a resonant microwave circuit with a microwave amplifier to provide the excitation.
U.S. Pat. No. 7,098,598 (Kraus et. al.) discloses capacitive coupling by optimizing the value of the coupling capacitance to the bulb. The optimization is done using specific ranges of thicknesses of the coupling capacitor dielectric in combination with a specific range of dielectric constant. This approach limits the size of the bulb into which radio-frequency (RF) energy can be coupled efficiently for a given operating frequency. In particular, it is more difficult to couple energy into smaller bulbs because of the smaller value of the coupling capacitance.
Each of the embodiments described above uses a dielectric or metal/dielectric waveguiding body forming—whether deliberately or unwittingly—a resonant cavity surrounding the bulb containing the plasma. The driving microwave energy is introduced into the waveguide body using various probing means well-known to those skilled in the art of microwave engineering. The waveguide body surrounding the bulb brings with it a host of difficulties including wasted light, poor etendue, lamp size related to resonance or excitation frequency, shift in resonant frequency during lamp start-up/warm-up requiring additional circuitry and complexity, manufacturing obstacles, and related costs. These obstacles, and the obstacles discussed above, including lifetime and size limitations, are overcome by the invention presented herein.